Leadless cardiac pacemaker system with conductive communication

ABSTRACT

A cardiac pacing system comprises multiple leadless cardiac pacemakers configured for implantation in electrical contact with a cardiac chamber and configured for multi-chamber cardiac pacing. The individual leadless cardiac pacemakers comprise at least two leadless electrodes configured for delivering cardiac pacing pulses, sensing evoked and/or natural cardiac electrical signals, and communicating bidirectionally among the leadless cardiac pacemaker plurality.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to and incorporatesherein by reference in its entirety for all purposes, Provisional U.S.Patent Application Nos.: 60/726,706 entitled “LEADLESS CARDIAC PACEMAKERWITH CONDUCTED COMMUNICATION,” filed Oct. 14, 2005; 60/761,531 entitled“LEADLESS CARDIAC PACEMAKER DELIVERY SYSTEM,” filed Jan. 24, 2006;60/729,671 entitled “LEADLESS CARDIAC PACEMAKER TRIGGERED BY CONDUCTEDCOMMUNICATION,” filed Oct. 24, 2005; 60/737,296 entitled “SYSTEM OFLEADLESS CARDIAC PACEMAKERS WITH CONDUCTED COMMUNICATION,” filed Nov.16, 2005; 60/739,901 entitled “LEADLESS CARDIAC PACEMAKERS WITHCONDUCTED COMMUNICATION FOR USE WITH AN IMPLANTABLECARDIOVERTER-DEFIBRILLATOR,” filed Nov. 26, 2005; 60/749,017 entitled“LEADLESS CARDIAC PACEMAKER WITH CONDUCTED COMMUNICATION AND RATERESPONSIVE PACING,” filed Dec. 10, 2005; and 60/761,740 entitled“PROGRAMMER FOR A SYSTEM OF LEADLESS CARDIAC PACEMAKERS WITH CONDUCTEDCOMMUNICATION,” filed Jan. 24,2006; all by Peter M. Jacobson.

BACKGROUND

Cardiac pacing electrically stimulates the heart when the heart'snatural pacemaker and/or conduction system fails to provide synchronizedatrial and ventricular contractions at appropriate rates and intervalsfor a patient's needs. Such bradycardia pacing provides relief fromsymptoms and even life support for hundreds of thousands of patients.Cardiac pacing may also give electrical overdrive stimulation intendedto suppress or convert tachyarrhythmias, again supplying relief fromsymptoms and preventing or terminating arrhythmias that could lead tosudden cardiac death.

Cardiac pacing is usually performed by a pulse generator implantedsubcutaneously or sub-muscularly in or near a patient's pectoral region.The generator usually connects to the proximal end of one or moreimplanted leads, the distal end of which contains one or more electrodesfor positioning adjacent to the inside or outside wall of a cardiacchamber. The leads have an insulated electrical conductor or conductorsfor connecting the pulse generator to electrodes in the heart. Suchelectrode leads typically have lengths of 50 to 70 centimeters.

A conventional pulse generator may be connected to more than oneelectrode-lead. For example, atrio-ventricular pacing, also commonlycalled dual-chamber pacing, involves a single pulse generator connectedto one electrode-lead usually placed in the right atrium and a secondelectrode-lead usually placed in the right ventricle. Such a system canelectrically sense heartbeat signals and deliver pacing pulsesseparately in each chamber. In typical use, the dual-chamber pacingsystem paces the atrium if no atrial heartbeat is sensed since apredetermined time, and then paces the ventricle if no ventricularheartbeat is sensed within a predetermined time after the natural orpaced atrial beat. Such pulse generators can also alter the timing ofatrial and ventricular pacing pulses when sensing a ventricular beatthat is not preceded by an atrial beat within a predetermined time; thatis, a ventricular ectopic beat or premature ventricular contraction.Consequently, dual-chamber pacing involves pacing and sensing in anatrium and a ventricle, and internal communication element so that anevent in either chamber can affect timing of pacing pulses in the otherchamber.

Recently, left-ventricular cardiac pacing has been practiced toameliorate heart failure; a practice termed cardiac resynchronizationtherapy (CRT). CRT has been practiced with electrode-leads and a pulsegenerator, either an implantable cardioverter-defibrillator (CRT-D) oran otherwise conventional pacemaker (CRT-P). The left-ventricular pacingconventionally uses an electrode in contact with cardiac muscle in thatchamber. The corresponding electrode-lead is usually placedendocardially in a transvenous manner through the coronary sinus vein,or epicardially. Left-ventricular pacing is usually practiced togetherwith right-atrial and right-ventricular pacing with a single implantedpulse generator connected to three electrode-leads. CRT pulse generatorscan independently vary the time between an atrial event andright-ventricular pacing, and the time between an atrial event andleft-ventricular pacing, so that the left ventricular pacing pulse canprecede, follow, or occur at the same time as the right-ventricularpacing pulse. Similarly to dual-chamber pacing, systems withleft-ventricular pacing also change atrial and ventricular pacing timingin response to premature ventricular contractions. Consequently, CRT-Dor CRT-P involves pacing in an atrium and in two ventricles, sensing inthe atrium and at least one ventricle, and an internal communicationelement so that an event in the atrium can affect timing of pacingpulses in each ventricle, and an internal communication element so thatan event in at least one ventricle can affect timing of pacing pulses inthe atrium and the other ventricle.

Pulse generator parameters are usually interrogated and modified by aprogramming device outside the body, via a loosely-coupled transformerwith one inductance within the body and another outside, or viaelectromagnetic radiation with one antenna within the body and anotheroutside.

Although tens of thousands of dual-chamber and CRT systems are implantedannually, several problems are known.

The pulse generator, when located subcutaneously, presents a bulge inthe skin that patients can find unsightly or unpleasant. Patients canmanipulate or “twiddle” the device. Even without persistent twiddling,subcutaneous pulse generators can exhibit erosion, extrusion, infection,and disconnection, insulation damage, or conductor breakage at the wireleads. Although sub-muscular or abdominal placement can address some ofthese concerns, a more difficult surgical procedure is involved forimplantation and adjustment, which can prolong patient recovery.

A conventional pulse generator, whether pectoral or abdominal, has aninterface for connection to and disconnection from the electrode leadsthat carry signals to and from the heart. Usually at least one maleconnector molding has at least one terminal pin at the proximal end ofthe electrode lead. The at least one male connector mates with at leastone corresponding female connector molding and terminal block within theconnector molding at the pulse generator. Usually a setscrew is threadedin at least one terminal block per electrode lead to secure theconnection electrically and mechanically. One or more O-rings usuallyare also supplied to help maintain electrical isolation between theconnector moldings. A setscrew cap or slotted cover is typicallyincluded to provide electrical insulation of the setscrew. The complexconnection between connectors and leads provides multiple opportunitiesfor malfunction.

For example, failure to introduce the lead pin completely into theterminal block can prevent proper connection between the generator andelectrode.

Failure to insert a screwdriver correctly through the setscrew slot,causing damage to the slot and subsequent insulation failure.

Failure to engage the screwdriver correctly in the setscrew can causedamage to the setscrew and preventing proper connection.

Failure to tighten the setscrew adequately also can prevent properconnection between the generator and electrode, however over-tighteningof the setscrew can cause damage to the setscrew, terminal block, orlead pin, and prevent disconnection if necessary for maintenance.

Fluid leakage between the lead and generator connector moldings, or atthe setscrew cover, can prevent proper electrical isolation.

Insulation or conductor breakage at a mechanical stress concentrationpoint where the lead leaves the generator can also cause failure.

Inadvertent mechanical damage to the attachment of the connector moldingto the generator can result in leakage or even detachment of themolding.

Inadvertent mechanical damage to the attachment of the connector moldingto the lead body, or of the terminal pin to the lead conductor, canresult in leakage, an open-circuit condition, or even detachment of theterminal pin and/or molding.

The lead body can be cut inadvertently during surgery by a tool, or cutafter surgery by repeated stress on a ligature used to hold the leadbody in position. Repeated movement for hundreds of millions of cardiaccycles can cause lead conductor breakage or insulation damage anywherealong the lead body.

Although leads are available commercially in various lengths, in someconditions excess lead length in a patient exists and is to be managed.Usually the excess lead is coiled near the pulse generator. Repeatedabrasion between the lead body and the generator due to lead coiling canresult in insulation damage to the lead.

Friction of the lead against the clavicle and the first rib, known assubclavian crush, can result in damage to the lead.

In dual-chamber pacing and CRT, multiple leads are implanted in the samepatient and sometimes in the same vessel. Abrasion between these leadsfor hundreds of millions of cardiac cycles can cause insulationbreakdown or even conductor failure.

Communication between the implanted pulse generator and externalprogrammer uses a telemetry coil or antenna and associated circuitry inthe pulse generator where complexity increases the size and cost of thedevices. Moreover, power necessary from the pulse generator battery forcommunication typically exceeds power for pacing by one or more ordersof magnitude, introducing a requirement for battery power capabilitythat can prevent selecting the most optimal battery construction for theotherwise low-power requirements of pacing.

SUMMARY

According to an embodiment of a cardiac pacing system, multiple leadlesscardiac pacemakers are configured for implantation in electrical contactwith a cardiac chamber and configured for multi-chamber cardiac pacing.The individual leadless cardiac pacemakers comprise at least twoleadless electrodes configured for delivering cardiac pacing pulses,sensing evoked and/or natural cardiac electrical signals, andcommunicating bidirectionally among the leadless cardiac pacemakerplurality.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention relating to both structure and method ofoperation may best be understood by referring to the followingdescription and accompanying drawings, in which similar referencecharacters denote similar elements throughout the several views:

FIG. 1A is a pictorial diagram showing an embodiment of a cardiac pacingsystem including multiple leadless cardiac pacemakers that can be usedin combination for multi-chamber cardiac pacing using conductivecommunication;

FIG. 1B is a schematic block diagram showing interconnection ofoperating elements of an embodiment of a leadless cardiac pacemaker thatcan be used in the multi-chamber cardiac pacing system;

FIG. 2 is a pictorial diagram showing the physical location of someelements of an embodiment of a leadless biostimulator that can be usedas part of a multi-chamber cardiac pacing system;

FIG. 3 is a pictorial diagram that depicts the physical location of someelements in an alternative embodiment of a leadless biostimulator thatcan be used as part of a multi-chamber cardiac pacing system;

FIG. 4 is a time waveform graph illustrating a conventional pacingpulse;

FIG. 5 is a time waveform graph depicting a pacing pulse adapted forcommunication as implemented for an embodiment of the illustrativepacing system;

FIG. 6 is a time waveform graph showing a sample pulse waveform usingoff-time variation for communication;

FIG. 7 is a state-mechanical representation illustrating an embodimentof a technique for operation of an atrial leadless cardiac pacemaker ina multi-chamber cardiac pacing system;

FIG. 8 is a state-mechanical representation illustrating an embodimentof a technique for operation of a right-ventricular leadless cardiacpacemaker in a multi-chamber cardiac pacing system;

FIG. 9 is a state-mechanical representation illustrating an embodimentof a technique for operation of a left-ventricular leadless cardiacpacemaker in a multi-chamber cardiac pacing system;

FIGS. 10A and 10B are schematic flow charts that depict embodiments ofmethods for operating an atrial leadless cardiac pacemaker inmulti-chamber cardiac pacing;

FIGUREs 11A and 11B are schematic flow charts that depict embodiments ofmethods for operating an right-ventricular leadless cardiac pacemaker inmulti-chamber cardiac pacing; and

FIGS. 12A and 12B are schematic flow charts that depict embodiments ofmethods for operating a left-ventricular leadless cardiac pacemaker inmulti-chamber cardiac pacing.

DETAILED DESCRIPTION

A system of leadless cardiac pacemakers with low-power conductedcommunication enables dual-chamber pacing, CRT-P, or other multi-chamberpacing.

Various embodiments of a system of leadless cardiac pacemakers withconducted communication for multi-chamber pacing are disclosed that canimplement, for example, dual-chamber pacing or three-chamber pacing forcardiac resynchronization therapy. The individual leadless cardiacpacemakers can be substantially enclosed in a hermetic housing suitablefor placement on or attachment to the inside or outside of a cardiacchamber. The pacemaker can have at least two electrodes located within,on, or near the housing, for delivering pacing pulses to and sensingelectrical activity from the muscle of the cardiac chamber, and forbidirectional communication with at least one other co-implantedleadless cardiac pacemaker and optionally with another device outsidethe body. The housing can contain a primary battery to provide power forpacing, sensing, and communication. The housing can also containcircuits for sensing cardiac activity from the electrodes, receivinginformation from at least one other device via the electrodes,generating pacing pulses for delivery via the electrodes, transmittinginformation to at least one other device via the electrodes, monitoringdevice health, and controlling these operations in a predeterminedmanner.

A cardiac pacing system includes two or more leadless cardiac pacemakersto enable improved performance in comparison to conventionalmulti-chamber cardiac pacing arrangements.

In some embodiments, a cardiac pacing system comprises two or moreleadless pacemakers for implantation adjacent to the inside or outsidewall of a cardiac chamber, without the need for a connection between thepulse generator and electrode lead that can be connected or disconnectedduring implantation and repair procedures, and without the need for alead body.

In some embodiments of a cardiac pacing system, communication betweenimplanted leadless cardiac pacemakers and optionally between animplanted leadless cardiac pacemaker and a device external to the body,uses conducted communication via the same electrodes used for pacing,without the need for an antenna or telemetry coil.

Some embodiments and/or arrangements can implement communication betweenan implanted leadless cardiac pacemaker and a device or devices internalor external to the body, with power requirements similar to those forcardiac pacing, to enable optimization of battery performance. Forexample, transmission from the leadless cardiac pacemaker adds no powerwhile reception adds a limited amount of power, such as about 25microwatts.

Referring to FIG. 1A, a pictorial diagram, which is not to scale, showsan embodiment of a cardiac pacing system 100 including multiple leadlesscardiac pacemakers 102 that can be used in combination for multi-chambercardiac pacing and can communicate via conductive communication. FIG. 1Bis a schematic block diagram showing an embodiment of a leadless cardiacpacemaker 102 that can be a component of the cardiac pacing system 100.In the system 100, multiple leadless cardiac pacemakers 102 areindividually configured for implantation in electrical contact withmultiple cardiac chambers 104 and arranged in combination formulti-chamber cardiac pacing. The individual leadless cardiac pacemakers102 comprise two or more leadless electrodes 108 configured fordelivering cardiac pacing pulses, sensing evoked and/or natural cardiacelectrical signals, and communicating bidirectionally among the leadlesscardiac pacemakers.

The illustrative cardiac pacing system 100 enables extended performancein comparison to conventional dual-chamber cardiac pacing and cardiacresynchronization therapy (CRT-P). The depicted cardiac pacing system100 can be configured for dual-chamber, CRT-P, and other multi-chambercardiac pacing schemes.

Individual pacemakers of the multiple leadless cardiac pacemakers 102can comprise a hermetic housing 110 configured for placement on orattachment to the inside or outside of the cardiac chambers 104. The twoor more leadless electrodes 108 proximal to the housing 110 can beconfigured for bidirectional communication with one or more otherdevices 106 within or outside the body.

The cardiac pacing system 100 can perform multi-chamber cardiac pacingin the absence of a pulse generator located in the pectoral region orabdomen of a patient, in the absence of an electrode-lead separate fromthe pulse generator, in the absence of a communication coil or antenna,and without imposing an additional requirement on battery power forcommunication of pacing pulse delivery.

The cardiac pacing system 100 attains improved performance through usageof at least two leadless cardiac pacemakers 102. An individual leadlesscardiac pacemaker 102 can be substantially enclosed in a hermetichousing 110 which is suitable for placement on or attachment to theinside or outside of a cardiac chamber 104. The pacemaker 102 has atleast two electrodes 108 located within, on, or near the housing 110,for delivering pacing pulses to and sensing electrical activity from themuscle of the cardiac chamber 104, and for bidirectional communicationwith at least one other leadless cardiac pacemaker within the body, andpossibly for bidirectional communication with at least one other device106 outside the body. The illustrative housing 110 contains a primarybattery 114 to supply power for pacing, sensing, and communication. Thedepicted housing 110 also contains circuits for sensing cardiac activityfrom the electrodes, receiving information from at least one otherdevice via the electrodes 108, generating pacing pulses for delivery viathe electrodes 108, transmitting information to at least one otherdevice via the electrodes 108, optionally monitoring device health, andcontrolling the operations in a predetermined manner.

FIG. 1B depicts a single leadless cardiac pacemaker 102 and shows thepacemaker's functional elements substantially enclosed in a hermetichousing 110. The pacemaker 102 has at least two electrodes 108 locatedwithin, on, or near the housing 110, for delivering pacing pulses to andsensing electrical activity from the muscle of the cardiac chamber, andfor bidirectional communication with at least one other device within oroutside the body. Hermetic feedthroughs 130, 131 conduct electrodesignals through the housing 110. The housing 110 contains a primarybattery 114 to supply power for pacing, sensing, and communication. Thehousing 110 also contains circuits 132 for sensing cardiac activity fromthe electrodes 108, circuits 134 for receiving information from at leastone other device via the electrodes 108, and a pulse generator 116 forgenerating pacing pulses for delivery via the electrodes 108 and alsofor transmitting information to at least one other device via theelectrodes 108. The housing 110 can further contain circuits formonitoring device health, for example a battery current monitor 136 anda battery voltage monitor 138, and can contain circuits 112 forcontrolling operations in a predetermined manner.

Individual pacemakers 102 of the leadless cardiac pacemaker pluralitycan be configured to intercommunicate and to communicate with anon-implanted programmer via the electrodes 108 that are also used fordelivering pacing pulses. Accordingly, the pacemakers 102 can beconfigured for antenna-less and telemetry coil-less communication. Theindividual pacemakers 102 can also intercommunicate among multiplepacemakers and communicate with a non-implanted programmer viacommunication that has outgoing communication power requirementsessentially met by power consumed in cardiac pacing.

The two or more leadless electrodes 108 can be configured to communicatebidirectionally among the multiple leadless cardiac pacemakers tocoordinate pacing pulse delivery using messages that identify an eventat an individual pacemaker originating the message. A pacemaker orpacemakers that receive the message react as directed by the messagedepending on the message origin or location. In some embodiments orconditions, the two or more leadless electrodes 108 can be configured tocommunicate bidirectionally among the multiple leadless cardiacpacemakers and transmit data including designated codes for eventsdetected or created by an individual pacemaker. Individual pacemakerscan be configured to issue a unique code corresponding to an event typeand a location of the sending pacemaker.

Information communicated on the incoming communication channel caninclude but is not limited to pacing rate, pulse duration, sensingthreshold, and other parameters commonly programmed externally inconventional pacemakers. Information communicated on the outgoingcommunication channel can include but is not limited to programmableparameter settings, pacing and sensing event counts, battery voltage,battery current, device health, and other information commonly displayedby external programmers used with conventional pacemakers. The outgoingcommunication channel can also echo information from the incomingchannel, to confirm correct programming.

Moreover, information communicated on the incoming channel can alsoinclude a message from another leadless cardiac pacemaker signifyingthat the other leadless cardiac pacemaker has sensed a heartbeat or hasdelivered a pacing pulse, and identifies the location of the otherpacemaker. Similarly, information communicated on the outgoing channelcan also include a message to another leadless cardiac pacemaker orpacemakers that the sending leadless cardiac pacemaker has sensed aheartbeat or has delivered a pacing pulse at the location of the sendingpacemaker.

For example, in some embodiments an individual pacemaker 102 of themultiple leadless cardiac pacemakers can be configured to deliver acoded pacing pulse with a code assigned according to pacemaker locationand configured to transmit a message to one or more other leadlesscardiac pacemakers via the coded pacing pulse wherein the codeidentifies the individual pacemaker originating an event. The pacemakeror pacemakers receiving the message are adapted to respond to themessage in a predetermined manner depending on type and location of theevent.

In some embodiments and in predetermined conditions, an individualpacemaker 102 of the multiple leadless cardiac pacemakers can beconfigured to communicate to one or more other implanted pacemakersindication of the occurrence of a sensed heartbeat at the individualpacemaker location via generation of a coded pacing pulse triggered bythe sensed heartbeat in a natural refractory period following the sensedheartbeat.

Multiple leadless cardiac pacemakers 102 can be configured forco-implantation in a single patient and multiple-chamber pacing, forexample by defining logic internal to the pacemakers 102 at manufacture,by programming using an external programmer, or the like. Bidirectionalcommunication among the multiple leadless cardiac pacemakers can bearranged to communicate notification of a sensed heartbeat or deliveredpacing pulse event and encoding type and location of the event toanother implanted pacemaker or pacemakers. The pacemaker or pacemakersreceiving the communication decode the information and respond dependingon location of the receiving pacemaker and predetermined systemfunctionality.

Also shown in FIG. 1B, the primary battery 114 has positive terminal 140and negative terminal 142. A suitable primary battery has an energydensity of at least 3 W·h/cc, a power output of 70 microwatts, a volumeless than 1 cubic centimeter, and a lifetime greater than 5 years.

One suitable primary battery uses beta-voltaic technology, licensed toBetaBatt Inc. of Houston, Tex., USA, and developed under a trade nameDEC™ Cell, in which a silicon wafer captures electrons emitted by aradioactive gas such as tritium. The wafer is etched in athree-dimensional surface to capture more electrons. The battery issealed in a hermetic package which entirely contains the low-energyparticles emitted by tritium, rendering the battery safe for long-termhuman implant from a radiological-health standpoint. Tritium has ahalf-life of 12.3 years so that the technology is more than adequate tomeet a design goal of a lifetime exceeding 5 years.

Current from the positive terminal 140 of primary battery 114 flowsthrough a shunt 144 to a regulator circuit 146 to create a positivevoltage supply 148 suitable for powering the remaining circuitry of thepacemaker 102. The shunt 144 enables the battery current monitor 136 toprovide the processor 112 with an indication of battery current drainand indirectly of device health.

The illustrative power supply can be a primary battery 114 such as abeta-voltaic converter that obtains electrical energy fromradioactivity. In some embodiments, the power supply can be selected asa primary battery 114 that has a volume less than approximately 1 cubiccentimeter.

In an illustrative embodiment, the primary battery 114 can be selectedto source no more than 70 microwatts instantaneously since a higherconsumption may cause the voltage across the battery terminals tocollapse. Accordingly in one illustrative embodiment the circuitsdepicted in FIG. 1B can be designed to consume no more than a total of64 microwatts. The design avoids usage of a large filtering capacitorfor the power supply or other accumulators such as a supercapacitor orrechargeable secondary cell to supply peak power exceeding the maximuminstantaneous power capability of the battery, components that would addvolume and cost.

In various embodiments, the system can manage power consumption to drawlimited power from the battery, thereby reducing device volume. Eachcircuit in the system can be designed to avoid large peak currents. Forexample, cardiac pacing can be achieved by discharging a tank capacitor(not shown) across the pacing electrodes. Recharging of the tankcapacitor is typically controlled by a charge pump circuit. In aparticular embodiment, the charge pump circuit is throttled to rechargethe tank capacitor at constant power from the battery.

Implantable systems that communicate via long distance radio-frequency(RF) schemes, for example Medical Implant Communication Service (MICS)transceivers, which exhibit a peak power specification on the order of10 milliwatts, and other RF or inductive telemetry schemes are unable tooperate without use of an additional accumulator. Moreover, even withthe added accumulator, sustained operation would ultimately cause thevoltage across the battery to collapse.

Referring to FIG. 2, a schematic pictorial view shows an embodiment ofthe leadless cardiac pacemaker 102 that can be used with at least oneother pacemaker in the cardiac pacing system 100. The leadless cardiacpacemaker 102 comprises a hermetic housing 110 configured for placementon or attachment to the inside or outside of a cardiac chamber 104. Twoor more electrodes 108 abut or are adjacent to the housing 110. Theelectrodes 108 are configured for delivering pacing pulses and receivingtriggering signals from the other pulse generator 106. The electrodes108 can also sense electrical activity from cardiac chamber muscle.

Furthermore, the electrodes 108 are adapted for bidirectionalcommunication with at least one other device within or outside the body.For example, the leadless pacemaker 102 can be configured to communicatewith a non-implanted programmer or one or more implanted pulsegenerators via the same electrodes 108 that are used for deliveringpacing pulses. The illustrative leadless pacemaker 102 is adapted forantenna-less and telemetry coil-less communication. Usage of theelectrodes 108 for communication enables the leadless pacemaker 102 tocommunicate with a non-implanted programmer or one or more implantedpulse generators via communication that adds nothing to powerrequirements in addition to power requirements for cardiac pacing. Forexample, transmission from the leadless cardiac pacemaker 102 adds nopower while reception adds on the order of 25 microwatts.

The illustrative example avoids usage of radiofrequency (RF)communication to send pacing instructions to remote electrodes on abeat-to-beat basis to cause the remote electrodes to emit a pacingpulse. RF communication involves use of an antenna andmodulation/demodulation unit in the remote electrode, which increaseimplant size significantly. Also, communication of pacing instructionson a beat-to-beat basis increases power requirements for the main bodyand the remote electrode. In contrast, the illustrative system andstimulator do not require beat-to-beat communication with anycontrolling main body.

The illustrative leadless pacemaker 102 includes an internal powersource that can supply all energy for operations and pulse generation.In contrast, some conventional implanted pulse generators have remotepacing electrodes that receive some or all energy from an energy sourcethrough an RF induction technique, an energy transfer scheme thatemploys a large loop antenna on the remote electrode that increases sizesignificantly. In addition, energy transfer with the RF inductiontechnique is inefficient and is associated with a significant increasein battery size of the energy source. In contrast, the illustrativeleadless pacemaker 102 uses an internal battery and does not requireenergy to be drawn from outside sources. Also in the conventionalsystem, the energy source receives sensing information by RFcommunication from the remote electrodes and sends pacing instructionsto the electrodes on a beat-to-beat basis in a configuration that usesan addressing scheme in which the identity of specific remote pacingelectrodes is stored in the energy source memory. The conventionalmethod can also be inefficient due to overhead for transmitting anidentification number from/to a generic pacing electrode at implantand/or during sensing. The illustrative leadless pacemaker 102 avoidssuch overhead through a structure in which pulse generationfunctionality is independent within a single implantable body.

Another conventional technology uses a system of addressable remoteelectrodes that stimulate body tissue without requiring a main body tosend commands for individual stimulations. The remote electrodes arespecified to be of a size and shape suitable for injection rather thanfor endocardial implantation. A controller can set operating parametersand send the parameters to remote electrodes by addressablecommunication, enabling the remote electrodes function relativelyautonomously while incurring some overhead to controller operations.However, the remote electrodes do not sense or monitor cardiacinformation and rely on the main body to provide sensing functionality.In contrast, the illustrative leadless pacemaker 102 combines pacing andsensing of intrinsic cardiac activity in a single implantable body.

In some embodiments, the controller 112 in one leadless cardiacpacemaker 102 can access signals on the electrodes 108 and can examineoutput pulse duration from another pacemaker for usage as a signaturefor determining triggering information validity and, for a signaturearriving within predetermined limits, activating delivery of a pacingpulse following a predetermined delay of zero or more milliseconds. Thepredetermined delay can be preset at manufacture, programmed via anexternal programmer, or determined by adaptive monitoring to facilitaterecognition of the triggering signal and discriminating the triggeringsignal from noise. In some embodiments or in some conditions, thecontroller 112 can examine output pulse waveform from another leadlesscardiac pacemaker for usage as a signature for determining triggeringinformation validity and, for a signature arriving within predeterminedlimits, activating delivery of a pacing pulse following a predetermineddelay of zero or more milliseconds.

The illustrative leadless pacemaker 102 has one or more structures thatenable fixture to tissue, for example suture holes 224, 225 or a helix226. The affixing structures enable implantation of the leadlesspacemaker 102 directly to the cardiac muscle and with ligatures inprocedures where the exterior surface of the heart can be accessed.

Also shown in FIG. 2, a cylindrical hermetic housing 110 is shown withannular electrodes 108 at housing extremities. In the illustrativeembodiment, the housing 110 can be composed of alumina ceramic whichprovides insulation between the electrodes. The electrodes 108 aredeposited on the ceramic, and are platinum or platinum-iridium.

Several techniques and structures can be used for attaching the housing110 to the interior or exterior wall of cardiac chamber muscle 104.

A helix 226 and slot 228 enable insertion of the device endocardially orepicardially through a guiding catheter. A screwdriver stylet can beused to rotate the housing 110 and force the helix 226 into muscle 104,thus affixing the electrode 108A in contact with stimulable tissue.Electrode 108B serves as an indifferent electrode for sensing andpacing. The helix 226 may be coated for electrical insulation, and asteroid-eluting matrix may be included near the helix to minimizefibrotic reaction, as is known in conventional pacing electrode-leads.

In other configurations, suture holes 224 and 225 can be used to affixthe device directly to cardiac muscle with ligatures, during procedureswhere the exterior surface of the heart is exposed.

Other attachment structures used with conventional cardiacelectrode-leads including tines or barbs for grasping trabeculae in theinterior of the ventricle, atrium, or coronary sinus may also be used inconjunction with or instead of the illustrative attachment structures.

Referring to FIG. 3, a pictorial view shows another embodiment of asingle leadless cardiac pacemaker 102 that can be used in a cardiacpacing system 100 with at least one other pacemaker. The leadlesscardiac pacemaker 102 includes a cylindrical metal housing 310 with anannular electrode 108A and a second electrode 108B. Housing 310 can beconstructed from titanium or stainless steel. Electrode 108A can beconstructed using a platinum or platinum-iridium wire and a ceramic orglass feed-thru to provide electrical isolation from the metal housing.The housing can be coated with a biocompatible polymer such as medicalgrade silicone or polyurethane except for the region outlined byelectrode 108B. The distance between electrodes 108A and 108B should beapproximately 1 cm to optimize sensing amplitudes and pacing thresholds.A helix 226 and slot 228 can be used for insertion of the deviceendocardially or epicardially through a guiding catheter. In addition,suture sleeves 302 and 303 made from silicone can be used to affix tothe device directly to cardiac muscle with ligatures, for example in anepicardial or other application.

Referring to FIG. 4, a typical output-pulse waveform for a conventionalpacemaker is shown. The approximately-exponential decay is due todischarge of a capacitor in the pacemaker through theapproximately-resistive load presented by the electrodes and leads.Typically the generator output is capacitor-coupled to one electrode toensure net charge balance. The pulse duration is shown as T0 and istypically 500 microseconds.

When the depicted leadless pacemaker 102 is used in combination with atleast one other pacemaker or other pulse generator in the cardiac pacingsystem 100 and is generating a pacing pulse but is not optionallysending data for communication, the pacing waveform of the leadlesspacemaker 102 can also resemble the conventional pacing pulse shown inFIG. 4.

Referring to FIG. 5, a time waveform graph depicts an embodiment of anoutput-pacing pulse waveform adapted for communication. The output-pulsewaveform of the illustrative leadless pacemaker 102 is shown during atime when the pacemaker 102 is optionally sending data for communicationand also delivering a pacing pulse, using the same pulse generator 116and electrodes 108 for both functions.

FIG. 5 shows that the pulse generator 102 has divided the output pulseinto shorter pulses 501, 502, 503, 504; separated by notches 505, 506,and 507. The pulse generator 102 times the notches 505, 506, and 507 tofall in timing windows W1, W2, and W4 designated 508, 509, and 511respectively. Note that the pacemaker 102 does not form a notch intiming window W3 designated 510. The timing windows are each shownseparated by a time T1, approximately 100 microseconds in the example.

As controlled by processor 112, pulse generator 116 selectivelygenerates or does not generate a notch in each timing window 508, 509,510, and 511 so that the device 102 encodes four bits of information inthe pacing pulse. A similar scheme with more timing windows can sendmore or fewer bits per pacing pulse. The width of the notches is small,for example approximately 15 microseconds, so that the delivered chargeand overall pulse width, specifically the sum of the widths of theshorter pulses, in the pacing pulse is substantially unchanged from thatshown in FIG. 4. Accordingly, the pulse shown in FIG. 5 can haveapproximately the same pacing effectiveness as that shown in FIG. 4,according to the law of Lapique which is well known in the art ofelectrical stimulation.

In the leadless cardiac pacemaker 102, a technique can be used toconserve power when detecting information carried on pacing pulses fromother implanted devices. The leadless cardiac pacemaker 102 can havemultiple gain settings on the receiving or sensing amplifier 132, forexample using a low-gain setting for normal operation. The low-gainsetting could be insufficiently sensitive to decode gated information ona pacing pulse accurately, but could detect whether the pacing pulse ispresent. If an edge of a pacing pulse is detected during low-gainoperation, the amplifier 132 can be switched quickly to the high-gainsetting, enabling the detailed encoded data to be detected and decodedaccurately. Once the pacing pulse has ended, the receiving amplifier 132can be set back to low-gain. In the illustrative technique, a usefulcondition is that the receiving amplifier 132 shift to the more accuratehigh-gain quickly when called upon. To allow a maximum amount of timefor shifting to occur, the encoded data can be placed at the end of thepacing pulse.

As an alternative or in addition to using notches in the stimulationpulse, the pulses can be generated with varying off-times, specificallytimes between pulses during which no stimulation occurs. The variationof off-times can be small, for example less than 10 milliseconds total,and can impart information based on the difference between a specificpulse's off-time and a preprogrammed off-time based on desired heartrate. For example, the device can impart four bits of information witheach pulse by defining 16 off-times centered around the preprogrammedoff-time. FIG. 6 is a graph showing a sample pulse generator outputwhich incorporates a varying off-time scheme. In the figure, time T_(p)represents the preprogrammed pulse timing. Time T_(d) is the delta timeassociated with a single bit resolution for the data sent by the pulsegenerator. The number of T_(d) time increments before or after themoment specified by T_(p) gives the specific data element transmitted.The receiver of the pulse generator's communication has advanceinformation of the time T_(p). The communication scheme is primarilyapplicable to overdrive pacing in which time T_(p) is not dynamicallychanging or altered based on detected beats.

An aspect of proper functionality for a cardiac pacemaker is maintenanceof a specified minimum internal supply voltage. When pacing tankcapacitor charging occurs, the supply voltage can drop from apre-charging level, a drop that can become more significant when thebattery nears an end-of-life condition and has reduced current sourcingcapability. Accordingly, in some implementations the leadless cardiacpacemaker 102 can be configured to stop charging the pacing tankcapacitor when the supply voltage drops below a specified level.Accordingly, the processor 112 can be configured to control rechargingof the tank capacitor so that recharging is discontinued when batteryterminal voltage falls below a predetermined value, ensuring sufficientvoltage for powering the leadless cardiac pacemaker circuitry. Whencharging ceases, the supply voltage returns to the value before chargingbegan. In other implementations, the charge current can be lowered toprevent the supply voltage from dropping below the specified level,possibly creating difficulty in ensuring the same pacing rate or pacingpulse amplitude since the lower charge current results in the pacingtank taking longer to reach the desired voltage level.

FIG. 5 depicts a technique in which information is encoded in notches inthe pacing pulse. FIG. 6 shows a technique of conveying information bymodulating the off-time between pacing pulses. Alternatively or inaddition to the two illustrative coding schemes, overall pacing pulsewidth can be used to impart information. For example, a paced atrialbeat may exhibit a pulse width of 500 microseconds and an intrinsicatrial contraction can be identified by reducing the pulse width by 30microseconds. Information can be encoded by the absolute pacing pulsewidth or relative shift in pulse width. Variations in pacing pulse widthcan be relatively small and have no impact on pacing effectiveness.

In some embodiments, a pacemaker 102 can use the leadless electrodes 108to communicate bidirectionally among multiple leadless cardiacpacemakers and transmit data including designated codes for eventsdetected or created by an individual pacemaker wherein the codes encodeinformation using pacing pulse width.

The illustrative scheme for transmitting data does not significantlyincrease the current consumption of the pacemaker. For example, thepacemaker could transmit data continuously in a loop, with noconsumption penalty.

The illustrative schemes for transmitting data enable assignment ofdesignated codes to events detected or caused by a leadless cardiacpacemaker, such as sensing a heartbeat or delivering a pacing pulse atthe location of the pacemaker that senses the event. Individual leadlesscardiac pacemakers 102 in a system 100 can be configured, either atmanufacture or with an external programmer as described above, to issuea unique code corresponding to the type of event and location of theleadless cardiac pacemaker. By delivery of a coded pacing pulse with acode assigned according to the pacemaker location, a leadless cardiacpacemaker can transmit a message to any and all other leadless cardiacpacemakers implanted in the same patient, where the code signifies theorigin of the event. Each other leadless cardiac pacemaker can reactappropriately to the conveyed information in a predetermined mannerencoded in the internal processor 112, as a function of the type andlocation of the event coded in the received pulse. A leadless cardiacpacemaker 102 can thus communicate to any and all other co-implantedleadless cardiac pacemakers the occurrence of a sensed heartbeat at theoriginating pacemaker's location by generating a coded pacing pulsetriggered by the sensed event. Triggered pacing occurs in the naturalrefractory period following the heartbeat and therefore has no effect onthe chamber where the leadless cardiac pacemaker is located.

Referring again to FIG. 1B, the circuit 132 for receiving communicationvia electrodes 108 receives the triggering information as described andcan also optionally receive other communication information, either fromthe other implanted pulse generator 106 or from a programmer outside thebody. This other communication could be coded with a pulse-positionscheme as described in FIG. 5 or could otherwise be a pulse-modulated orfrequency-modulated carrier signal, preferably from 10 kHz to 100 kHz.The illustrative scheme of a modulated carrier is applicable not only tointercommunication among multiple implanted pacemakers but also isapplicable to communication from an external programmer.

The illustrative leadless pacemaker 102 could otherwise receivetriggering information from the other pulse generator 106 implantedwithin the body via a pulse-modulated or frequency-modulated carriersignal, instead of via the pacing pulses of the other pulse generator106.

With regard to operating power requirements in the leadless cardiacpacemaker 102, for purposes of analysis, a pacing pulse of 5 volts and 5milliamps amplitude with duration of 500 microseconds and a period of500 milliseconds has a power requirement of 25 microwatts.

In an example embodiment of the leadless pacemaker 102, the processor112 typically includes a timer with a slow clock that times a period ofapproximately 10 milliseconds and an instruction-execution clock thattimes a period of approximately 1 microsecond. The processor 112typically operates the instruction-execution clock only briefly inresponse to events originating with the timer, communication amplifier134, or cardiac sensing amplifier 132. At other times, only the slowclock and timer operate so that the power requirement of the processor112 is no more than 5 microwatts.

For a pacemaker that operates with the aforementioned slow clock, theinstantaneous power consumption specification, even for acommercially-available micropower microprocessor, would exceed thebattery's power capabilities and would require an additional filtercapacitor across the battery to prevent a drop of battery voltage belowthe voltage necessary to operate the circuit. The filter capacitor wouldadd avoidable cost, volume, and potentially lower reliability.

For example, a microprocessor consuming only 100 microamps would requirea filter capacitor of 5 microfarads to maintain a voltage drop of lessthan 0.1 volt, even if the processor operates for only 5 milliseconds.To avoid the necessity for such a filter capacitor, an illustrativeembodiment of a processor can operate from a lower frequency clock toavoid the high instantaneous power consumption, or the processor can beimplemented using dedicated hardware state machines to supply a lowerinstantaneous peak power specification.

In a pacemaker, the cardiac sensing amplifier typically operates with nomore than 5 microwatts. A communication amplifier at 100 kHz operateswith no more than 25 microwatts. The battery ammeter and batteryvoltmeter operate with no more than 1 microwatt each.

A pulse generator typically includes an independent rate limiter with apower consumption of no more than 2 microwatts.

The total power consumption of the pacemaker is thus 64 microwatts, lessthan the disclosed 70-microwatt battery output.

Improvement attained by the illustrative cardiac pacing system 100 andleadless cardiac pacemaker 102 is apparent.

In a specific embodiment, the outgoing communication power requirementplus the pacing power requirement does not exceed approximately 25microwatts. In other words, outgoing communication adds essentially nopower to the power used for pacing.

The illustrative leadless cardiac pacemaker 102 can have sensing andprocessing circuitry that consumes no more than 10 microwatts as inconventional pacemakers.

The described leadless cardiac pacemaker 102 can have an incomingcommunication amplifier for receiving triggering signals and optionallyother communication which consumes no more than 25 microwatts.

Furthermore, the leadless cardiac pacemaker 102 can have a primarybattery that exhibits an energy density of at least 3 watt-hours percubic centimeter (W·h/cc).

In an illustrative application of the cardiac pacing system 100,multiple leadless cardiac pacemakers 102 can be co-implanted in a singlepatient to provide a system for dual-chamber pacing, CRT-P, or any othermulti-chamber pacing application. Each leadless cardiac pacemaker in thesystem can use the illustrative communication structures to communicatethe occurrence of a sensed heartbeat or a delivered pacing pulse at thelocation of sensing or delivery, and a communication code can beassigned to each combination of event type and location. Each leadlesscardiac pacemaker can receive the transmitted information, and the codeof the information can signify that a paced or sensed event has occurredat another location and indicate the location of occurrence. Thereceiving leadless cardiac pacemaker's processor 112 can decode theinformation and respond appropriately, depending on the location of thereceiving pacemaker and the desired function of the system. FIGS. 7 and8 are state diagrams that illustrate application of illustrativecombined control operations in an atrial and right-ventricular leadlesscardiac pacemaker respectively, to implement a simple dual-chamberpacing system when co-implanted. FIG. 9 is a state diagram thatillustrates inclusion of a left-ventricular leadless cardiac pacemakerto form a CRT-P system.

In various embodiments, each leadless cardiac pacemaker may also encodeother information destined for co-implanted leadless cardiac pacemakers,besides markers of paced or sensed events.

For clarity of illustration, descriptions of the atrial, rightventricular, and left-ventricular leadless cardiac pacemakers inrespective FIGS. 7, 8, and 9 show only basic functions of eachpacemaker. Other functions such as refractory periods, fallback modeswitching, algorithms to prevent pacemaker-mediated tachycardia, and thelike, can be added to the leadless cardiac pacemakers and to the systemin combination. Also for clarity, functions for communication with anexternal programmer are not shown and are shown elsewhere herein.

Referring to FIG. 7, a state-mechanical representation shows operationof a leadless cardiac pacemaker for implantation adjacent to atrialcardiac muscle. As explained above, a leadless cardiac pacemaker can beconfigured for operation in a particular location and system either atmanufacture or by an external programmer. Similarly, all individualpacemakers of the multiple pacemaker system can be configured foroperation in a particular location and a particular functionality atmanufacture and/or at programming by an external programmer wherein“configuring” means defining logic such as a state machine and pulsecodes used by the leadless cardiac pacemaker.

In a cardiac pacing system, the multiple leadless cardiac pacemakers cancomprise an atrial leadless cardiac pacemaker implanted in electricalcontact to an atrial cardiac chamber. The atrial leadless cardiacpacemaker can be configured or programmed to perform several controloperations 700 in combination with one or more other pacemakers. In await state 702 the atrial leadless cardiac pacemaker waits for anearliest occurring event of multiple events including a sensed atrialheartbeat 704, a communication of an event sensed on the at least twoleadless electrodes encoding a pacing pulse marking a heartbeat 706 at aventricular leadless cardiac pacemaker, or timeout of an interval timedlocally in the atrial leadless cardiac pacemaker shown as escapeinterval timeout 708. The atrial pacemaker responds to a sensed atrialheartbeat 704 by generating 710 an atrial pacing pulse that signals toone or more other pacemakers that an atrial heartbeat has occurred,encoding the atrial pacing pulse with a code signifying an atriallocation and a sensed event type. The atrial pacing pulse can be encodedusing the technique shown in FIG. 5 with a unique code signifying thelocation in the atrium. After pacing the atrium, the atrial cardiacpacemaker times 712 a predetermined atrial-to-atrial (AA) escapeinterval. Accordingly, the atrial leadless cardiac pacemaker restartstiming 712 for a predetermined escape interval, called the AA (atrial toatrial) escape interval, which is the time until the next atrial pacingpulse if no other event intervenes. The atrial leadless cardiacpacemaker then re-enters the Wait state 702. The atrial pacemaker alsoresponds to timeout of a first occurring escape interval 708 bydelivering an atrial pacing pulse 710, causing an atrial heartbeat withthe atrial pacing pulse encoding paced type and atrial location of anatrial heartbeat event. When the atrial escape interval times out, shownas transition 708, the atrial leadless cardiac pacemaker delivers anatrial pacing pulse. Because no other atrial heartbeat has occurredduring the duration of the escape interval, the atrial pacing pulse doesnot fall in the atria's natural refractory period and therefore shouldeffectively pace the atrium, causing an atrial heartbeat. The atrialpacing pulse, coded in the manner shown in FIG. 5, also signals to anyand all other co-implanted leadless cardiac pacemakers that an atrialheartbeat has occurred. If functionality is enhanced for a more complexsystem, the atrial leadless cardiac pacemaker can use a different codeto signify synchronous pacing triggered by an atrial sensed event incomparison to the code used to signify atrial pacing at the end of anescape interval. However, in the simple example shown in FIGS. 7 and 8,the same code can be used for all atrial pacing pulses. In fact, for thesimple dual-chamber pacing system described in FIGS. 7 and 8 encodingmay be omitted because each leadless cardiac pacemaker can conclude thatany detected pacing pulse, which is not generated locally, must haveoriginated with the other co-implanted leadless cardiac pacemaker. Aftergenerating the atrial pacing pulse 710, the atrial leadless cardiacpacemaker starts timing an atrial (AA) escape interval at action 712,and then returns to the wait state 702.

The atrial leadless cardiac pacemaker can further operate in response toanother pacemaker. The atrial pacemaker can detect 706 a signaloriginating from a co-implanted ventricular leadless cardiac pacemaker.The atrial pacemaker can examine the elapsed amount of theatrial-to-atrial (AA) escape time interval since a most recent atrialheartbeat and determine 714 whether the signal originating from theco-implanted ventricular leadless cardiac pacemaker is premature. Thus,if the atrial leadless cardiac pacemaker detects a signal originatingfrom a co-implanted ventricular leadless cardiac pacemaker, shown assensed ventricular pacing 706, then the atrial device examines theamount of the escape interval elapsed since the last atrial heartbeat atdecision point 714 to determine whether the ventricular event is“premature”, meaning too late to be physiologically associated with thelast atrial heartbeat and in effect premature with respect to the nextatrial heartbeat. In the absence 716 of a premature signal, the atrialpacemaker waits 702 for an event with no effect on atrial pacing. Incontrast if the signal is premature 718, the pacemaker restarts 720 aventricle-to-atrium (VA) escape interval that is shorter than theatrial-to-atrial (AA) escape interval and is representative of a typicaltime from a ventricular beat to a next atrial beat in sinus rhythm,specifically the atrial interval minus the atrio-ventricular conductiontime. After starting 720 the VA interval, the atrial leadless cardiacpacemaker returns to wait state 702, whereby a ventricular prematurebeat can be said to “recycle” the atrial pacemaker. The pacemakerresponds to timeout of the atrial-to-atrial (AA) escape interval 708 bydelivering an atrial pacing pulse 710, causing an atrial heartbeat. Theatrial pacing pulse encodes the paced type and atrial location of anatrial heartbeat event.

The atrial leadless cardiac pacemaker can be further configured to timea prolonged post-ventricular atrial refractory period (PVARP) afterrecycling in presence of the premature signal, thereby preventingpacemaker-mediated tachycardia (PMT). Otherwise, if a receivedventricular pacing signal evaluated at decision point 714 is not foundto be premature, then the atrial leadless cardiac pacemaker followstransition 716 and re-enters the wait state 702 without recycling, thuswithout any effect on the timing of the next atrial pacing pulse.

Referring to FIG. 8, a state-mechanical representation depicts operationof a leadless cardiac pacemaker for implantation adjacent toright-ventricular cardiac muscle. The leadless cardiac pacemaker can beconfigured for operation in a particular location and system either atmanufacture or by an external programmer. A system comprising multipleleadless cardiac pacemakers can include a right-ventricular leadlesscardiac pacemaker implanted in electrical contact to a right-ventricularcardiac chamber. The right-ventricular leadless cardiac pacemaker can beconfigured to perform actions 800 for coordinated pacing in combinationwith the other pacemakers. The right-ventricular leadless cardiacpacemaker waits 802 for the earliest occurring event of multiple eventsincluding a sensed right-ventricular heartbeat 804, a sensedcommunication of a pacing pulse 806 marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout 808 of an escape interval.Generally, the sensed communication of a pacing pulse 806 can be anysuitable sensed communication of an event originating at anotherco-implanted leadless cardiac pacemaker, in the illustrative embodimenta pacing pulse marking a heartbeat at an atrial leadless cardiacpacemaker shown as sensed atrial pacing. The escape interval timeout 808can be any suitable timeout of an interval timed locally in theright-ventricular leadless cardiac pacemaker.

The right-ventricular leadless cardiac pacemaker responds to the sensedright-ventricular heartbeat 804 by generating 810 a right-ventricularpacing pulse that signals to at least one other pacemaker of themultiple cardiac pacemakers that a right-ventricular heartbeat hasoccurred. Thus, when a sensed right-ventricular heartbeat occurs 804,the right-ventricular leadless cardiac pacemaker generates 810 aright-ventricular pacing pulse, not to pace the heart but rather tosignal to another leadless cardiac pacemaker or pacemakers that aright-ventricular heartbeat has occurred. The right-ventricular pacingpulse can be encoded with a code signifying the right-ventricularlocation and a sensed event type. The right-ventricular pacing pulse iscoded in the manner shown in FIG. 5 with a unique code signifying thelocation in the right ventricle. Upon right-ventricular pacing pulsegeneration 810, the right-ventricular leadless cardiac pacemaker cantime 812 a predetermined right ventricular-to-right ventricular (VV)escape interval. The right-ventricular leadless cardiac pacemakerrestarts 812 timing of a predetermined escape interval, called the VV(right-ventricular to right-ventricular) escape interval, which is thetime until the next right-ventricular pacing pulse if no other eventintervenes. The VV escape interval is started after delivering aventricular pacing pulse subsequent to various events including anatrial-ventricular (AV) delay, a ventricular-to-ventricular (VV) delay,or a ventricular sensed event.

The right-ventricular leadless cardiac pacemaker can further beconfigured to set the ventricular-to-ventricular (VV) escape intervallonger than a predetermined atrial-to-atrial (AA) escape interval toenable backup ventricular pacing at a low rate corresponding to the VVescape interval in case of failure of a triggered signal from aco-implanted atrial leadless cardiac pacemaker. Typically, the VV(right) escape interval is longer than the AA interval depicted in FIG.7, so that the system supports backup ventricular pacing at a relativelylow rate in case of failure of the co-implanted atrial leadless cardiacpacemaker. In normal operation of the system, timeout of the VV intervalnever occurs. The right-ventricular leadless cardiac pacemaker thenre-enters the Wait state 802.

The right-ventricular leadless cardiac pacemaker can respond to timeoutof a first occurring escape interval 808 by delivering 810 a rightventricular pacing pulse, causing a right ventricular heartbeat. Theright ventricular pacing pulse can encode information including pacedtype and right-ventricular location of a right ventricular heartbeatevent.

When the right-ventricular escape interval times out 808, theright-ventricular leadless cardiac pacemaker delivers 810 aright-ventricular pacing pulse. Because no other right-ventricularheartbeat has occurred during the duration of the VV escape interval,the pacing pulse 810 does not fall in the ventricles' natural refractoryperiod and therefore should effectively pace the ventricles, causing aventricular heartbeat. The right-ventricular pacing pulse, coded in themanner shown in FIG. 5, also signals to any and all other co-implantedleadless cardiac pacemakers that a right-ventricular heartbeat hasoccurred. If useful for the function of a more complex system, theright-ventricular leadless cardiac pacemaker can use a different code tosignify synchronous pacing triggered by a right-ventricular sensed eventin comparison to the code used to signify right-ventricular pacing atthe end of a VV escape interval. However, in the simple example shown inFIGS. 7 and 8, the same code can be used for all right-ventricularpacing pulses. In fact, for the simple dual-chamber pacing systemdescribed in FIGS. 7 and 8, a code may be omitted because each leadlesscardiac pacemaker can conclude that any detected pacing pulse which isnot generated local to the pacemaker originates with the otherco-implanted leadless cardiac pacemaker. After generating 810 theright-ventricular pacing pulse, the right-ventricular leadless cardiacpacemaker starts timing 812 a right-ventricular escape interval VV, andthen returns to the wait state 802.

The right-ventricular leadless cardiac pacemaker can further beconfigured to detect 806 a signal originating from a co-implanted atrialleadless cardiac pacemaker. The right-ventricular leadless cardiacpacemaker examines the elapsed amount of the ventricular-to-ventricular(VV) escape interval since a most recent right-ventricular heartbeat anddetermines 814 whether the signal originating from the co-implantedatrial leadless cardiac pacemaker is premature. An atrial event isdefined as premature if too early to trigger an atrio-ventricular delayto produce a right-ventricular heartbeat. In the presence of a prematuresignal 816, the right-ventricular leadless cardiac pacemaker returns tothe wait state 802 with no further action. Thus, a premature atrial beatdoes not affect ventricular pacing. In the absence of a premature signal818, the right-ventricular leadless cardiac pacemaker starts 820 a rightatrium to right ventricular (AV) escape interval that is representativeof a typical time from an atrial beat to a right-ventricular beat insinus rhythm. Thus a non-premature atrial event leads to starting 820 anAV (right) atrium to right-ventricular escape interval that represents atypical time from an atrial beat to a right-ventricular beat innormally-conducted sinus rhythm. After starting 820 the AV interval, theright-ventricular leadless cardiac pacemaker returns to the wait state802 so that a non-premature atrial beat can “trigger” theright-ventricular pacemaker after a physiological delay. Theright-ventricular leadless cardiac pacemaker also responds to timeout ofeither the VV escape interval and the AV escape interval 808 bydelivering 810 a right ventricular pacing pulse, causing a rightventricular heartbeat. The right ventricular pacing pulse encodes pacedtype and right-ventricular location of a right ventricular heartbeatevent.

Accordingly, co-implanted atrial and right-ventricular leadless cardiacpacemakers depicted in FIGS. 7 and 8 cooperate to form a dual-chamberpacing system.

Referring to FIG. 9, a state-mechanical representation illustrates theoperation of a leadless cardiac pacemaker for implantation adjacent toleft-ventricular cardiac muscle. The left-ventricular cardiac pacemakercan be used in combination with the dual-chamber pacemaker that includesatrial leadless cardiac pacemaker and the right-ventricular leadlesscardiac pacemaker described in FIGS. 7 and 8 respectively to form asystem for CRT-P. A leadless cardiac pacemaker, for example theleft-ventricular cardiac pacemaker, can be configured for operation in aparticular location and system either at manufacture or by an externalprogrammer.

A cardiac pacing system, such as a CRT-P system, can include multipleleadless cardiac pacemakers including a left-ventricular leadlesscardiac pacemaker implanted in electrical contact to a left-ventricularcardiac chamber. The left-ventricular leadless cardiac pacemaker canexecute operations of an illustrative pacing method 900. In a wait state902, the left-ventricular cardiac pacemaker waits 902 at theleft-ventricular leadless cardiac pacemaker for an earliest occurringevent of multiple events including a sensed communication 904 of apacing pulse marking a heartbeat at an atrial leadless cardiac pacemakerand timeout 906 of a left ventricular escape interval. Generally, thesensed communication 904 can be the sensed communication of an eventoriginating at another co-implanted leadless cardiac pacemaker, in theillustrative embodiment a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker shown as sensed atrial pacing. The escapeinterval timeout 906 can be timeout of an interval timed locally in theleft-ventricular leadless cardiac pacemaker. In the wait state 902 forthe left-ventricular leadless cardiac pacemaker, operation is simplifiedand the left-ventricular pacemaker does not respond to left-ventricularheartbeats. Also, the left-ventricular cardiac pacemaker does not pacethe left ventricle in the absence of a triggering signal from the atrialleadless cardiac pacemaker. The left-ventricular cardiac pacemakerresponds to timeout 906 of the left ventricular escape interval bydelivering 908 a left ventricular pacing pulse, causing a leftventricular heartbeat. The left ventricular pacing pulse encodes thetype and location of a left ventricular heartbeat event. Theleft-ventricular pacing pulse can be coded in the manner shown in FIG. 5to communicate signals to any and all other co-implanted leadlesscardiac pacemakers that a left-ventricular heartbeat has occurred,although such encoding is not necessary in the simplified CRT-P systemshown in the described embodiment because the other leadless cardiacpacemakers do not react to left-ventricular pacing. After generating 908the left-ventricular pacing pulse, the left-ventricular leadless cardiacpacemaker returns to the wait state 902.

The left-ventricular leadless cardiac pacemaker can be furtherconfigured detect a signal originating from a co-implanted atrialleadless cardiac pacemaker and examine the elapsed amount of the leftventricular escape interval since a most recent left-ventricularheartbeat. The left-ventricular cardiac pacemaker can determine 910whether the signal originating from the co-implanted atrial leadlesscardiac pacemaker is premature. If the left-ventricular leadless cardiacpacemaker detects sensed atrial pacing, then the left-ventricular devicedetermines whether the atrial event is premature, meaning too early totrigger an atrio-ventricular delay to produce a left-ventricularheartbeat. In the presence of a premature signal 912, theleft-ventricular cardiac pacemaker reverts to the wait state 902 andwaits for an event with no effect on ventricular pacing so that apremature atrial beat does not affect ventricular pacing. In absence ofa premature signal 914, the left-ventricular cardiac pacemaker starts916 a left atrium to left ventricular (AV) escape interval that isrepresentative of a typical time from an atrial beat to a leftventricular beat in normally-conducted sinus rhythm. As shown in thedepicted embodiment, the AV (left) escape interval can have a differentvalue from the AV (right) escape interval. After starting 916 the AVinterval, the left-ventricular leadless cardiac pacemaker returns towait state 902. Accordingly, a non-premature atrial beat can “trigger”the left-ventricular pacemaker after a physiological delay.

The left-ventricular cardiac pacemaker also responds to timeout 906 ofthe AV escape interval by delivering 908 a left ventricular pacingpulse, causing a left ventricular heartbeat. The left ventricular pacingpulse encodes paced type and left ventricular location of a leftventricular heartbeat event.

In various embodiments, the multiple leadless cardiac pacemakers cancomprise a right ventricular leadless cardiac pacemaker and a leftventricular leadless cardiac pacemaker that are configured to operatewith atrio-ventricular (AV) delays whereby a left ventricular pacingpulse can be delivered before, after, or substantially simultaneouslywith a right ventricular pacing pulse. For example, multipleco-implanted leadless cardiac pacemakers that function according to thestate diagrams shown in FIGS. 7, 8, and 9 can support CRT-P withleft-ventricular pacing delivered before, at the same time as, or afterright-ventricular pacing.

In various embodiments, multiple co-implanted leadless cardiacpacemakers can be configured for multi-site pacing that synchronizesdepolarization for tachyarrhythmia prevention.

Referring to FIGS. 10A and 10B, schematic flow charts illustrate anembodiment of a method for operating an atrial leadless cardiacpacemaker in multi-chamber cardiac pacing. FIG. 10A depicts a method1000 for multi-chamber cardiac pacing comprising configuring 1002 amultiple leadless cardiac pacemakers for implantation and configuring1004 an atrial leadless cardiac pacemaker of the multiple leadlesscardiac pacemakers for implantation in electrical contact to an atrialcardiac chamber. The atrial leadless cardiac pacemaker waits 1006 for anearliest occurring event of multiple events including a sensed atrialheartbeat, a communication of an event sensed on the at least twoleadless electrodes encoding a pacing pulse marking a heartbeat at aventricular leadless cardiac pacemaker, and timeout of anatrial-to-atrial (AA) escape interval. The atrial leadless cardiacpacemaker responds 1008 to the sensed atrial heartbeat by generating anatrial pacing pulse that signals to at least one pacemaker of themultiple leadless cardiac pacemakers that an atrial heartbeat hasoccurred and that encodes the atrial pacing pulse with a code signifyingan atrial location and a sensed event type. After either a sensed atrialheartbeat or timeout of an escape interval, the atrial leadless cardiacpacemaker delivers 1010 an atrial pacing pulse, causing an atrialheartbeat and starts 1012 timing a predetermined length AA escapeinterval, then waiting 1006 for an event. The atrial pacing pulseidentifies paced type and/or atrial location of an atrial heartbeatevent.

In some embodiments, the atrial leadless cardiac pacemaker can encode anatrial pacing pulse that identifies synchronous pacing triggered by anatrial sensed event with a first code and encode an atrial pacing pulsethat identifies atrial pacing following the AA escape interval with asecond code distinct from the first code.

In some embodiments or conditions, the atrial leadless cardiac pacemakercan deliver the atrial pacing pulse in absence of encoding whereby, fordual-chamber cardiac pacing, a pacing pulse that is not generated in afirst cardiac pacemaker that senses the pacing pulse is necessarilygenerated in a second cardiac pacemaker. Accordingly, neither the use ofa code to identify the chamber corresponding to a pacing pulse, nor theuse of a code to identify the type of pulse (whether paced or sensed) isa necessary step in a simple system such as a dual chamber pacing systemdisclosed in the specification.

The atrial leadless cardiac pacemaker can, upon delivery of an atrialpacing pulse, time an atrial-to-atrial (AA) escape interval.

FIG. 10B is a flow chart showing another aspect of an embodiment of amethod 1050 for operating an atrial leadless cardiac pacemaker. Theatrial leadless cardiac pacemaker detects 1052 a signal originating froma co-implanted ventricular leadless cardiac pacemaker and examines 1054an elapsed amount of the atrial-to-atrial (AA) escape interval since amost recent atrial heartbeat, determining 1056 whether the signaloriginating from the co-implanted ventricular leadless cardiac pacemakeris premature. In absence of a premature signal 1058, the atrial leadlesscardiac pacemaker waits 1060 for an event with no effect on atrialpacing, returning to wait state 1006. In presence of a premature signal1062, the atrial leadless cardiac pacemaker restarts 1064 aventricle-to-atrium (VA) escape interval that is shorter than theatrial-to-atrial (AA) escape interval and representative of a typicaltime from a ventricular beat to a next atrial beat in sinus rhythm, thenreturns to wait state 1006.

Referring to FIGS. 11A and 11B, schematic flow charts illustrate anembodiment of a method for operating a right-ventricular leadlesscardiac pacemaker in multi-chamber cardiac pacing. FIG. 11A depicts amethod 1100 for multi-chamber cardiac pacing comprising configuring 1102a plurality of leadless cardiac pacemakers for implantation andconfiguring 1104 a right-ventricular leadless cardiac pacemaker of themultiple leadless cardiac pacemakers for implantation in electricalcontact to a right-ventricular cardiac chamber. The right-ventricularleadless cardiac pacemaker waits 1106 for an earliest occurring event ofa several events including a sensed right-ventricular heartbeat, asensed communication of a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout of an escape interval. Theright-ventricular leadless cardiac pacemaker responds 1108 to the sensedright-ventricular heartbeat by generating a right-ventricular pacingpulse that signals to at least one pacemaker of the leadless cardiacpacemakers that a right-ventricular heartbeat has occurred and thatencodes the right-ventricular pacing pulse with a code signifying aright-ventricular location and a sensed event type. Theright-ventricular leadless cardiac pacemaker responds 1110 to timeout ofa first occurring escape interval by delivering a right ventricularpacing pulse, causing a right ventricular heartbeat, with the rightventricular pacing pulse encoding paced type and right ventricularlocation of a right ventricular heartbeat event, and times 1112 apredetermined ventricular-to-ventricular (VV) escape interval.

In some embodiments, the right-ventricular leadless cardiac pacemakercan encode a right-ventricular pacing pulse that identifies synchronouspacing triggered by a right-ventricular sensed event with a first codeand encode a right-ventricular pacing pulse that identifiesright-ventricular pacing following a ventricular-to-ventricular (VV)escape interval with a second code distinct from the first code.

In some embodiments, the right-ventricular leadless cardiac pacemaker,upon delivery of a right-ventricular pacing pulse, can time aventricular-to-ventricular (VV) escape interval.

FIG. 11B is a flow chart showing another aspect of an embodiment of amethod 1150 for operating a right-ventricular leadless cardiacpacemaker. The right-ventricular leadless cardiac pacemaker detects 1152a signal originating from a co-implanted atrial leadless cardiacpacemaker, examines 1154 the elapsed amount of theventricular-to-ventricular (VV) escape interval since a most recentright-ventricular heartbeat, and determines 1156 whether the signaloriginating from the co-implanted atrial leadless cardiac pacemaker ispremature. In presence 1158 of a premature signal, the right-ventricularleadless cardiac pacemaker waits 1160 for an event with no effect onventricular pacing, returning to wait state 1106. In absence 1162 of apremature signal, the right-ventricular leadless cardiac pacemakerstarts 1164 a right atrium to right ventricular (AV) escape intervalthat is representative of a typical time from an atrial beat to aright-ventricular beat in sinus rhythm, and then returns to the waitstate 1106.

Referring to FIGS. 12A and 12B, schematic flow charts illustrateembodiments of a method for operating a left-ventricular leadlesscardiac pacemaker in multi-chamber cardiac pacing. FIG. 12A depicts amethod 1200 for multi-chamber cardiac pacing comprising configuring 1202a plurality of leadless cardiac pacemakers for implantation andconfiguring 1204 a left-ventricular leadless cardiac pacemaker of theleadless cardiac pacemaker plurality for implantation in electricalcontact to a left-ventricular cardiac chamber and for operation incardiac resynchronization therapy (CRT-P). The left-ventricular cardiacpacemaker waits 1206 at the left-ventricular leadless cardiac pacemakerfor an earliest occurring event of a plurality of events comprising asensed communication of a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker and timeout of a left ventricular escapeinterval. The left-ventricular cardiac pacemaker responds 1208 totimeout of the left ventricular escape interval by delivering a leftventricular pacing pulse, causing a left ventricular heartbeat, the leftventricular pacing pulse encoding type and location of a leftventricular heartbeat event.

In some embodiments, the left-ventricular cardiac pacemaker canconfigure the left-ventricular leadless cardiac pacemaker for operationin cardiac resynchronization therapy (CRT-P).

FIG. 12B is a flow chart showing another aspect of an embodiment of amethod 1250 for operating a left-ventricular leadless cardiac pacemaker.The left-ventricular leadless cardiac pacemaker detects 1252 a signaloriginating from a co-implanted atrial leadless cardiac pacemaker,examines 1254 the elapsed amount of the left ventricular escape intervalsince a most recent left-ventricular heartbeat, and determines 1256whether the signal originating from the co-implanted atrial leadlesscardiac pacemaker is premature. In the presence 1258 of a prematuresignal, the left-ventricular cardiac pacemaker waits 1260 for an eventwith no effect on ventricular pacing. In the absence 1262 of a prematuresignal, the left-ventricular cardiac pacemaker starts 1264 a left atriumto left ventricular (AV) escape interval that is representative of atypical time from an atrial beat to a left ventricular beat in sinusrhythm.

Terms “substantially”, “essentially”, or “approximately”, that may beused herein, relate to an industry-accepted tolerance to thecorresponding term. Such an industry-accepted tolerance ranges from lessthan one percent to twenty percent and corresponds to, but is notlimited to, component values, integrated circuit process variations,temperature variations, rise and fall times, and/or thermal noise. Theterm “coupled”, as may be used herein, includes direct coupling andindirect coupling via another component, element, circuit, or modulewhere, for indirect coupling, the intervening component, element,circuit, or module does not modify the information of a signal but mayadjust its current level, voltage level, and/or power level. Inferredcoupling, for example where one element is coupled to another element byinference, includes direct and indirect coupling between two elements inthe same manner as “coupled”.

While the present disclosure describes various embodiments, theseembodiments are to be understood as illustrative and do not limit theclaim scope. Many variations, modifications, additions and improvementsof the described embodiments are possible. For example, those havingordinary skill in the art will readily implement the steps necessary toprovide the structures and methods disclosed herein, and will understandthat the process parameters, materials, and dimensions are given by wayof example only. The parameters, materials, and dimensions can be variedto achieve the desired structure as well as modifications, which arewithin the scope of the claims. Variations and modifications of theembodiments disclosed herein may also be made while remaining within thescope of the following claims. For example, although the description hassome focus on the pacemaker, system, structures, and techniques canotherwise be applicable to other uses, for example multi-site pacing forprevention of tachycardias in the atria or ventricles. Phraseology andterminology employed herein are for the purpose of the description andshould not be regarded as limiting. With respect to the description,optimum dimensional relationships for the component parts are to includevariations in size, materials, shape, form, function and manner ofoperation, assembly and use that are deemed readily apparent and obviousto one of ordinary skill in the art and all equivalent relationships tothose illustrated in the drawings and described in the specification areintended to be encompassed by the present description. Therefore, theforegoing is considered as illustrative only of the principles ofstructure and operation. Numerous modifications and changes will readilyoccur to those of ordinary skill in the art whereby the scope is notlimited to the exact construction and operation shown and described, andaccordingly, all suitable modifications and equivalents may be included.

What is claimed is:
 1. A cardiac pacing system comprising: a pluralityof leadless cardiac pacemakers individually configured for implantationin electrical contact with a cardiac chamber and configured incombination for multi-chamber cardiac pacing, the leadless cardiacpacemaker plurality individually comprising at least two leadless pacingelectrodes and a controller operatively connected to the electrodes andconfigured to deliver cardiac pacing pulses through the electrodes,sense evoked and/or natural cardiac electrical signals through theelectrodes, and bidirectionally communicate information through theelectrodes for coordinating pacing among the plurality of leadlesscardiac pacemakers.
 2. The system according to claim 1 wherein: thecontroller of at least one pacemaker of the plurality of leadlesscardiac pacemakers is further configured to communicate with anon-implanted programmer via the at least two pacing electrodes usingantenna-less and telemetry coil-less communication.
 3. A systemaccording to claim 1 wherein: individual pacemakers of the plurality ofleadless cardiac pacemakers comprise: a hermetic housing configured forplacement on or attachment to the inside or outside of a cardiacchamber; and the at least two leadless pacing electrodes proximal to thehousing configured for bidirectional communication with at least oneother device within or outside the body.
 4. The system according toclaim 1 wherein: the at least two leadless pacing electrodes areconfigured to communicate bidirectionally among the plurality ofleadless cardiac pacemakers to coordinate pacing pulse delivery usingmessages that identify an event at an individual pacemaker originatingthe message and pacemakers receiving the message react as directed bythe message depending on the message origin.
 5. The system according toclaim 1 wherein: the at least two leadless pacing electrodes areconfigured to communicate bidirectionally among the plurality ofleadless cardiac pacemakers and transmit data including designated codesfor events detected or created by an individual pacemaker, theindividual pacemakers configured to issue a unique code that identifiesan event type and a location of the individual pacemaker.
 6. The systemaccording to claim 1 wherein: individual pacemakers of the plurality ofleadless cardiac pacemakers are configured to deliver at least one pulseencoded with a code assigned according to pacemaker location andconfigured to transmit a message to at least one pacemaker of theplurality of leadless cardiac pacemakers via the encoded at least onepulse wherein the code identifies the individual pacemaker originatingan event, the at least one pacemaker receiving the message being adaptedto respond to the message in a predetermined manner depending on typeand location of the event.
 7. The system according to claim 1 wherein:individual pacemakers of the plurality of leadless cardiac pacemakersare configured to communicate to at least one pacemaker of the pluralityof leadless cardiac pacemakers occurrence of a sensed heartbeat at theindividual pacemaker location via generation of at least one coded pulsetriggered by a sensed heartbeat in a natural refractory period followingthe sensed heartbeat.
 8. The system according to claim 1 wherein: theplurality of leadless cardiac pacemakers is configured forco-implantation in a single patient and multiple-chamber pacing, thebidirectional communication among the plurality of leadless cardiacpacemakers adapted to communicate notification of a sensed heartbeat ordelivered pacing pulse event to at least one pacemaker of the pluralityof leadless cardiac pacemakers, the at least one pacemaker that receivesthe communication adapted to decode the information and responddepending on location of the receiving pacemaker and predeterminedsystem functionality.
 9. The system according to claim 1 wherein: theplurality of leadless cardiac pacemakers comprises an atrial leadlesscardiac pacemaker adapted for implantation in electrical contact to anatrial cardiac chamber, the atrial leadless cardiac pacemaker configuredto: wait for an earliest occurring event of a plurality of eventscomprising a sensed atrial heartbeat, a communication of an event sensedon the at least two leadless pacing electrodes encoding at least onepulse generated by a ventricular pulse generator and marking a heartbeatat a ventricular leadless cardiac pacemaker, or timeout of an escapeinterval; respond to the sensed atrial heartbeat by generating at leastone pulse via an atrial pulse generator that signals to at least onepacemaker of the plurality of leadless cardiac pacemakers that an atrialheartbeat has occurred and encoding at least one pulse generated by anatrial pulse generator with a code signifying an atrial location and asensed event type; time a predetermined atrial-to-atrial (AA) escapeinterval; and respond to timeout of an escape interval by delivering anatrial pacing pulse, causing an atrial heartbeat, and encoding at leastone pulse generated by the atrial pulse generator with informationdesignating paced type and atrial location of an atrial heartbeat event.10. The system according to claim 9 wherein: the atrial leadless cardiacpacemaker is configured to time an atrial-to-atrial (AA) escape intervalafter generating an atrial pacing pulse.
 11. The system according toclaim 9 wherein: the atrial leadless cardiac pacemaker is configured to:detect a signal originating from a co-implanted ventricular leadlesscardiac pacemaker; examine an elapsed amount of the atrial-to-atrial(AA) escape interval since a most recent atrial heartbeat; determinewhether the signal originating from the co-implanted ventricularleadless cardiac pacemaker is premature; in absence of a prematuresignal, wait for an event with no effect on atrial pacing; in presenceof a premature signal, restart a ventricle-to-atrial (VA) escapeinterval that is shorter than the atrial-to-atrial (AA) escape intervaland representative of a typical time from a ventricular beat to a nextatrial beat in sinus rhythm; and respond to timeout of the VA escapeinterval or the AA escape interval by delivering an atrial pacing pulse,causing an atrial heartbeat, encoding at least one pulse generated bythe atrial pulse generator with information designating paced type andatrial location of an atrial heartbeat event, and starting an AA escapeinterval and returning to a wait state.
 12. The system according toclaim 11 wherein: the atrial leadless cardiac pacemaker is furtherconfigured to: time a prolonged post-ventricular atrial refractoryperiod (PVARP) after recycling in presence of the premature signalwhereby pacemaker-mediated tachycardia is prevented.
 13. The systemaccording to claim 1 wherein: the plurality of leadless cardiacpacemakers comprise a right-ventricular leadless cardiac pacemakeradapted for implantation in electrical contact to a right-ventricularcardiac chamber, the right-ventricular leadless cardiac pacemakerconfigured to: wait for an earliest occurring event of a plurality ofevents comprising a sensed right-ventricular heartbeat, a sensedcommunication of at least one pulse marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout of an escape interval; respondto the sensed right-ventricular heartbeat by generating at least onepulse by a right-ventricular pulse generator that signals to at leastone pacemaker of the plurality of leadless cardiac pacemakers that aright-ventricular heartbeat has occurred and encoding the at least onepulse generated by the right-ventricular pulse generator with a codesignifying a right-ventricular location and a sensed event type; restarta predetermined right ventricular-to-right ventricular (VV) escapeinterval after delivering a ventricular pacing pulse after either anatrial ventricular (AV) delay, a ventricular to ventricular (VV) delay,or a ventricular sensed event; and respond to timeout of an escapeinterval by delivering at least one pulse generated by a rightventricular pulse generator, causing a right ventricular heartbeat, andencoding the right ventricular pacing pulse with information designatingpaced type and right-ventricular location of a right ventricularheartbeat event.
 14. The system according to claim 13 wherein: theright-ventricular leadless cardiac pacemaker is configured to: detect asignal originating from a co-implanted atrial leadless cardiacpacemaker; examine an elapsed amount of the ventricular-to-ventricular(VV) escape interval since a most recent right-ventricular heartbeat;determine whether the signal originating from the co-implanted atrialleadless cardiac pacemaker is premature; in presence of a prematuresignal, wait for an event with no effect on ventricular pacing; inabsence of a premature signal, start a right atrium to right ventricular(AV) escape interval that is representative of a typical time from anatrial beat to a right-ventricular beat in sinus rhythm; and respond totimeout of the VV escape interval or the AV escape interval bydelivering at least one pulse by the right ventricular pulse generator,causing a right ventricular heartbeat, encoding the at least one pulsegenerated by the right ventricular pulse generator with informationdesignating paced type and right-ventricular location of a rightventricular heartbeat event, and starting a ventricular-to-ventricular(VV) escape interval and returning to a wait state.
 15. The systemaccording to claim 13 wherein: the right-ventricular leadless cardiacpacemaker is configured to: set the ventricular-to-ventricular (VV)escape interval longer than a predetermined atrial-to-atrial (AA) escapeinterval to enable backup ventricular pacing at a low rate correspondingto the VV escape interval in case of failure of a triggered signal froma co-implanted atrial leadless cardiac pacemaker.
 16. The systemaccording to claim 1 wherein: the plurality of leadless cardiacpacemakers comprise a left-ventricular leadless cardiac pacemakerimplanted in electrical contact to a left-ventricular cardiac chamber,the left-ventricular leadless cardiac pacemaker configured to: wait atthe left-ventricular leadless cardiac pacemaker for an earliestoccurring event of a plurality of events comprising a sensedcommunication of a pacing pulse marking a heartbeat at an atrialleadless cardiac pacemaker, and timeout of a left ventricular escapeinterval; and respond to timeout of the left ventricular escape intervalby delivering a left ventricular pacing pulse, causing a leftventricular heartbeat, and encoding at least one pulse generated by aleft ventricular pulse generator with information designating type andlocation of a left ventricular heartbeat event.
 17. The system accordingto claim 16 wherein: the left-ventricular leadless cardiac pacemaker isconfigured to: detect a signal originating from a co-implanted atrialleadless cardiac pacemaker; examine an elapsed amount of the leftventricular escape interval since a most recent left-ventricularheartbeat; determine whether the signal originating from theco-implanted atrial leadless cardiac pacemaker is premature; in presenceof a premature signal, wait for an event with no effect on ventricularpacing; in absence of a premature signal, start a left atrium to leftventricular (AV) escape interval that is representative of a typicaltime from an atrial beat to a left ventricular beat in sinus rhythm; andrespond to timeout of the AV escape interval by delivering a leftventricular pacing pulse, causing a left ventricular heartbeat, encodingat least one pulse generated by the left ventricular pulse generatorwith information designating paced type and left ventricular location ofa left ventricular heartbeat event, and starting aventricular-to-ventricular (VV) escape interval and returning to a waitstate.
 18. The system according to claim 1 further comprising: acontroller coupled to the at least two pacing electrodes adapted toexamine output pulse duration from the at least one pacemaker of theplurality of leadless cardiac pacemakers for usage as a signature fordetermining triggering information validity and, for a signaturearriving within predetermined limits, activating delivery of a pacingpulse following a predetermined delay of zero or more milliseconds, thepredetermined delay being determined from a method in a group consistingof preset at manufacture, programmed via an external programmer, andadaptively monitoring and conforming to an interval between a priorevent of at least one predetermined type and the triggering signal. 19.The system according to claim 1 further comprising: a controller coupledto the at least two pacing electrodes adapted to examine output pulsewaveform from the at least one pacemaker of the plurality of leadlesscardiac pacemakers for usage as a signature for determining triggeringinformation validity and, for a signature arriving within predeterminedlimits, activating delivery of a pacing pulse following a predetermineddelay of zero or more milliseconds.
 20. The system according to claim 1wherein: individual pacemakers of the plurality of leadless cardiacpacemakers are configured for operation in a particular location and aparticular functionality at manufacture and/or at programming by anexternal programmer.
 21. The system according to claim 1 wherein: theplurality of leadless cardiac pacemakers comprise a right ventricularleadless cardiac pacemaker and a left ventricular leadless cardiacpacemaker configured to operate with atrio-ventricular (AV) delayswhereby a left ventricular pacing pulse can be delivered before, after,or substantially simultaneously with a right ventricular pacing pulse.22. The system according to claim 1 wherein: the plurality of leadlesscardiac pacemakers is configured for multi-site pacing that synchronizesdepolarization for tachyarrhythmia prevention.
 23. The system accordingto claim 1 wherein: the at least two leadless pacing electrodes areconfigured to communicate bidirectionally among the plurality ofleadless cardiac pacemakers and transmit data including designated codesfor events detected or created by an individual pacemaker wherein dataare encoded as pulse width.
 24. The system according to claim 1 wherein:the at least two leadless pacing electrodes are configured tocommunicate bidirectionally among the plurality of leadless cardiacpacemakers and transmit data including designated codes for eventsdetected or created by an individual pacemaker wherein data are encodedas binary-coded notches in a pacing pulse.
 25. The system according toclaim 1 further comprising: a receiving amplifier/filter adapted formultiple controllable gain settings; and a processor configured tocontrol gain setting for the receiving amplifier/filter, invoking alow-gain setting for normal operation and detecting presence of a pulse,and invoking a high-gain setting for detecting and decoding informationencoded in the detected pulse.
 26. The system according to claim 1further comprising: the at least two leadless pacing electrodesconfigured to communicate bidirectionally among the plurality ofleadless cardiac pacemakers and transmit data including designated codesfor events detected or created by an individual pacemaker wherein dataare encoded as modulation of off-time between pulses.
 27. The systemaccording to claim 1 further comprising: a tank capacitor selectivelyconnected to deliver pulses to a pair of the at least two leadlesspacing electrodes and adapted for charging and discharging wherein apacing pulse is generated; a charge pump circuit coupled to the tankcapacitor and adapted for controlling charging of the tank capacitor;and a processor configured to control recharging of the tank capacitorwherein recharging is discontinued when a battery terminal voltage fallsbelow a predetermined value to ensure sufficient voltage for poweringthe leadless cardiac pacemaker.